With an optical network employing optical branching/insertion or optical cross connection or the like, a great number of optical paths are set between multiple transmission nodes and multiple reception nodes. Therefore, multiple optical signals having mutually different combinations of transmission node and reception node may be transmitted using the same wavelength. Accordingly, it is difficult for a reception node to determine, just by detecting the wavelength of a received optical signal, whether or not the optical signal thereof has been transmitted from a desired transmission node, that is, whether or not the received optical signal is a desired optical signal.
Therefore, in order to enable a reception node to confirm a transmission source of the received optical signal, a node ID that indicates which transmission node a principal signal has been transmitted from may be superimposed on a principal signal thereof as an auxiliary signal. Also, in order to enable a relay node to identify which path is an optical path where the optical signal is transmitted from, the path ID of the optical path may be superimposed on the principal signal as the auxiliary signal.
As a technique to superimpose an auxiliary signal on a principal signal, there is a technique to superimpose an auxiliary signal on a principal signal using frequency modulation, that is, to subject a principal signal to frequency modulation using an auxiliary signal. More specifically, with this technique, a center frequency of the principal signal is slightly changed by “−Δf” or “+Δf” in response to “0” or “1” of digital information of the auxiliary signal. For example, in the event that a transmission frequency band width of the principal signal is several tens of GHz, the magnitude of Δf is around 0.1 to 0.5 GHz. At a reception node or relay node, a received optical signal is subjected to filtering using a wavelength filter, transmitted light after filtering is received at a photodiode to convert this into an electric signal, thereby converting the auxiliary signal superimposed on the principal signal into an intensity modulated signal. Specifically, the auxiliary signal superimposed on the principal signal by frequency modulation of ±Δf is detected by the filtering at the wavelength filter as a signal of which the intensity is changed by the worth according to ±Δf. In this manner, at a reception node or relay node, of the principal signal and auxiliary signal included in the optical signal, even without demodulating the principal signal, the auxiliary signal alone is demodulated.
The following non-patent literatures are examples of the related art of the present disclosure.
Tanimura, Takahito; Hoshida, Takeshi; Nakashima, Hisao; Akiyama, Yuichi; Yan, Meng; Tao, Zhenning; Rasmussen, Jens C., “In-Band FSK Supervisory Signaling Between Adaptive Optical Transceivers Employing Digital Signal Processing”, in Proceeding of ECOC 2011, We.7.A, September 2011.
Tanimura, Takahito; Hoshida, Takeshi; Oda, Shoichiro; Akiyama, Yuichi; Nakashima, Hisao; Aoki, Yasuhiko; Cao, Yinwen; Yan, Meng; Tao, Zhenning; Rasmussen, Jens C., “Superimposition and Detection of Frequency Modulated Tone for Light Path Tracing Employing Digital Signal Processing and Optical Filter”, in Proceeding of OFC 2012, OW4G.4, March 2012.
In order to demodulate the auxiliary signal superimposed on the principal signal by frequency modulation using a wavelength filter as described above, it is desirable to set near the center frequency of the principal signal, that is, near the center wavelength of the principal signal as the center wavelength of the wavelength filter.
However, in the event that precision of a light-emitting element that a transmission node includes to generate the principal signal is inferior, or the like, the center wavelength of the principal signal may be inaccurate by involuntarily deviating from a nominal wavelength. Also, the center wavelength of a laser diode to be sometimes used as a light-emitting element included in a transmission node is shifted to a long wavelength side as operation temperature thereof increases. Therefore, along with increase in operation temperature of the laser diode, the center wavelength of the principal signal involuntarily deviates from a nominal wavelength.
According to the center wavelength of the principal signal involuntarily deviating from a nominal wavelength, it becomes difficult to fixedly set the optimal position on the wavelength axis of the wavelength filter as to the principal signal. Therefore, heretofore, demodulation precision of the auxiliary signal superimposed on the principal signal by frequency modulation has sometimes deteriorated.